Light micrograph of thin section through a shell (test) of a Baculogypsina spaerulata foraminifera, or sand star. The test was mounted in embedding media and ground down to the equatorial plane of the test. Here you can see the initial spiralled chamber in the middle of the image, surrounded by several lateral chambers. Foraminifera are marine single-celled protozoa that construct and inhabit shells composed of several chambers. These are usually penetrated by pores through which the cellular content is extruded. The shells form an important component of chalk. In past geological ages, foraminifera were so numerous that their shells, largely composed of calcium carbonate, have formed immense fossil deposits, seen today as limestone. Magnification: 10x when printed at 10 centimetres across.